In the excavation of an oil well, a number of pipes called casing are embedded in the well and thus the wall of the well is prevented from crumbling. In the excavation of a well, a hole is excavated by drilling until a certain depth is reached, and thereafter, a casing is inserted into the excavated well in order to prevent the crumbling of the wall of the well. In this way, the well is excavated by successively continuing the drilling operation; however, the casing, to be embedded when the excavation proceeds to reach the next stage depth, is inserted downward through the previously embedded casing, so that the diameter of the casing to be embedded afterward in a deeper portion is required to be made smaller than the diameter of the previously embedded casing.
In an oil well thus excavated, the diameter of the casing in the upper portion of the well is large, and the casing becomes smaller in diameter with increasing depth, finally through which a steel pipe, which is called tubing, for oil or gas production is inserted. Consequently, the diameter of the casing in the upper portion of the well is designed by backward calculation from the tubing diameter to be ensured when the well is excavated to a predetermined depth.
Accordingly, when a deep well is excavated, the size of the casing in the upper portion becomes large and the cost required for excavation is thereby increased.
As described in the patent document 1, a design is made in which by radially expanding the casings in the well, the diameter difference between each pair of successive casings forming the multistage casing structure is made smaller, and consequently, the size of the upper portion of the well is made smaller. This method is a method in which a steel pipe having a diameter smaller than the required diameter of a steel pipe is inserted in an oil well, and the pipe is subjected to expanding working in the oil well so as to have a outside diameter required for the steel pipe. By adopting this method, as described above, the diameter of the casing in the upper portion of the well can be suppressed to be smaller, and the cost required for well excavation can thereby be reduced.
When a steel pipe is expanded in an oil well, the steel pipe still in a state subjected to expanding working is exposed to the environment of the produced fluid such as oil and gas. Consequently, the steel pipe still in a state subjected to expanding working is required to have predetermined performances. This is because it is impossible to apply a heat treatment over the whole length of the steel pipes after expanding working for the purpose of improving the characteristics thereof.
Pipes for oil wells are shipped in a state subjected to heat treatment, and conventionally, the corrosion resistance, and among others, the resistance to the sulfide stress cracking (hereinafter referred to as “SSC” as the case may be) in the environment of wet hydrogen sulfide, namely, the sulfide stress cracking resistance (hereinafter referred to as “SSC resistance” as the case may be) are taken into account. However, for steel pipes to which the expanding working technique is applied, it is particularly important to consider the SSC resistance degradation due to the working hardening caused by expansion.
In the patent document 2, a steel pipe is proposed in which the SSC resistance after being subjected to the expanding working is ensured. However, the steel pipe presented therein is a steel pipe in which, because the SSC resistance after expanding working is affected by the crystal grains and the strength of the steep pipe before expanding working, the crystal grain size is made to be a predetermined value or less in a manner associated with the strength, and hence for the steel pipe, the SSC resistance after expanding working is ensured.
However, for the production of such a steel pipe disclosed in the above-mentioned document, an appropriate heat treatment for forming fine grains is indispensable, and the control of such a heat treatment is not an easy task. Additionally, in the patent document 2, there is no description on the relation between the N in steel, particularly, the soluble N (nitrogen) and the diffusive hydrogen largely affecting the SSC generation.
Patent Document 1:                Japanese Publication of International Patent Application No. 7-507610.        
Patent Document 2:                Publication of unexamined Japanese Patent application No. 2002-266055.        